1. Field of the Invention
The present invention relates to an imaging apparatus, such as a video camera, having an automatic focusing device for automatically focusing on an object.
2. Description of the Related Art
In recent years, video apparatuses including video cameras or electronic cameras have been remarkably developed. In particular, functions, such as automatic focus (AF) or automatic exposure (AE), have come to be installed as standard functions to improve the functions and ease of operation thereof.
In one main-stream focusing method, utilizing an automatic focusing device, the sharpness of a picture is detected from among video signals obtained by an image sensor or the like by photoelectrically converting an object image, and the position of the lens for adjusting the focus (hereinafter referred to as a focus lens) is controlled so that maximum sharpness is attained.
To evaluate the sharpness, generally this method uses the intensity of high-frequency components of the video signal extracted by a band-pass filter (BPF) or the detected intensity of blur width (the width of an edge portion of the object) of a video signal extracted by a differentiation circuit or the like.
In a case where an ordinary object is photographed, the level of high-frequency components is low and the blur width is large when the object is not in focus. As the object comes into focus, the level of the high-frequency components becomes high, and the blur width becomes small. When the object is completely brought into focus, the level of the high-frequency components assumes a maximum value, and the blur width assumes a minimum value. Therefore, when the sharpness is low, the focus lens is driven as quickly as possible in the direction in which sharpness increases. As the sharpness increases, the speed at which the focus lens is driven is lowered, and the focus lens is stopped precisely at the peak of the hill representing sharpness. Such a control method is generally called the hill-climbing autofocus method (hill-climbing AF).
Since such an automatic focusing device described above is adopted, ease of operation is increased remarkably particularly in a video camera or the like in which a moving image is photographed, and the automatic focusing device has lately become indispensable.
In such a camera for converting an optical image into an electrical signal by an image sensor, the central portion of a picture is generally that area from which the sharpness necessary for hill climbing AF is to be extracted (hereinafter referred to as a focusing frame). Also, a method in which a focusing frame is varied depending upon the state of the object is known. According to this method, it is possible to bring an object located even at an end position of the picture into focus by, for example, enlarging the focusing frame.
Some automatic exposure control devices have functions for setting a shutter speed and an aperture value at predetermined values when a given object is photographed. This function is generally called a program AE. Program AE modes include a portrait mode, in which the depth of field is made shallow, so that people, in particular, or the like are photographed clearly, a sand and snow mode in which the brightness of people in a skiing area in winter or seashore in summer is intensified, and the like. The program AE function has come to be installed in most video apparatuses in order to easily obtain a more clear image.
In addition, the digitalization of circuits for processing image signals has progressed. Various new functions, for example, a close-up function for enlarging a part of a picture, have come to be routinely installed.
However, in such conventional video cameras equipped with a program AE, for example, in the case of the portrait mode of the program AE, a person's face is considered to be the prevalent object, but even if the face is in focus, the sharpness does not become very high. Therefore, when there is an object with high sharpness in the background, if such AF is performed as described above, in which a focusing frame is varied, a problem arises, for example, in that the focusing frame is automatically made larger and the focus would be brought on the background located at the edge portion of the picture.
Also, in the case of the close-up mode of the program AE, when the focusing frame is large, focus is made on an object outside of the picture, and no focus is made on the picture, which is problematical.
Automatic exposure control for realizing such a program AE is performed as described below.
FIG. 1 is a block diagram illustrating the construction of an automatic exposure control device used in the above-described conventional video camera.
Referring to FIG. 1; reference numeral 1 denotes a photographic lens; reference numeral 2 denotes as an iris for adjusting the amount of incident light which passes through the photographic lens 1; reference numeral 3 denotes an image sensor, such as a CCD, for photoelectrically converting an image which is formed by the photographic lens 1 on its image pickup plane and whose amount of light is adjusted by the iris 2; reference numeral 4 denotes a correlated double sampling circuit (CDS circuit) for reducing noise of an accumulated charge of the image sensor 3; reference numeral 5 denotes an AGC circuit for adjusting the gain of an image signal outputted from the correlated double sampling circuit 4; reference numeral 6 denotes a camera signal processing circuit for performing a predetermined signal processing on image signals outputted from the AGC circuit 5 in order to convert the signals into standardized video signals; reference numeral 7 denotes a gate circuit for dividing the picked-up picture into a plurality of pictures and gating a signal outputted from the AGC circuit 5 in order to extract image signals corresponding to a desired area; reference numeral 8 denotes an integrator for integrating each of the image signals corresponding to the inside of a specified area on the picture, which image signals are selected by the gate circuit 7, and determining the average light amount thereof; reference numeral 9 denotes an A/D converter for converting signals outputted from the integrator 8 into digital signals having a form for allowing a system control circuit 17, which will be described later, to process the signals; reference numeral 10 denotes a CCD drive circuit for controlling an accumulation operation, a reading operation and a resetting operation of the image sensor 3; reference numeral 11 denotes an iris motor for driving the iris 2; reference numeral 12 denotes an iris drive circuit for driving the iris motor 11; reference numeral 13 denotes a D/A converter for converting digital iris control signals outputted from the system control circuit 17 into analog signals; reference numeral 14 denotes an iris encoder formed of a Hall element for detecting the amount of opening of the iris 2, i.e., the aperture value, or the like; reference numeral 15 denotes an A/D converter for converting an output from the iris encoder 14 into digital signals having a form for allowing the system control circuit 17 to be able to process the signals; reference numeral 16 denotes a D/A converter for converting digital AGC control signals outputted from the system control circuit 17 into analog control signals, and supplying the signals to the AGC circuit 5; and reference numeral 17 denotes a system control circuit formed of a microcomputer for centrally controlling the entire video camera system.
In this construction, exposure control is performed by the iris 2 and the AGC circuit 5, i.e., two types of exposure control means. Also, the shutter speed is set to the same as one cycle (field period) of a vertical synchronization signal of a standard television signal (1/60 seconds in NTSC, and 1/50 second in PAL).
An explanation will be given below of specific operations of the automatic exposure control device in the video camera constructed as described above.
The amount of incident light which passes through the photographic lens 1 is controlled by the iris 2, and an optical image is formed on the image pickup plane of the image sensor 3 and photoelectrically converted by the image sensor 3. Noise in the image signals outputted from the image sensor 3 is removed by the CDS circuit 4, and the gain thereof is controlled by the AGC circuit 5. The resultant image signals are supplied to the camera signal processing circuit 6, and outputted as normalized television signals.
Next, the exposure control system for controlling exposure will be considered: luminance signals in the video signals outputted from the AGC circuit 5 are gated by the gate circuit 7; luminance signals corresponding to the inside of the focusing frame on the picture, indicating the range in which exposure control is performed, are extracted; and these signals are integrated by the integrator 8 to determine the average value, and supplied to the system control circuit 17 through the A/D converter 9. The system control circuit 17 controls the iris drive circuit 12 so that the luminance level received from the A/D converter 9 falls below a predetermined range, and controls the drive current supplied to the iris motor 11, thereby varying the aperture of the iris 2.
More specifically, when the exposure control system based on iris control is considered, a closed loop is formed such that the aperture on the iris 2 is controlled so that the luminance level of the video signals obtained by photoelectrically converting incident light which passes through the iris 2 is always kept within a predetermined range.
Also, when the exposure control system by the AGC circuit 5 is considered, a closed loop is formed, similar to the exposure control system based on the iris 2, such that the gain of the AGC circuit 5 is controlled by the system control circuit 17 so that the luminance level received by the system control circuit 17 falls below a predetermined range.
Next, actual control characteristics will be explained in more detail. FIG. 2 is a block diagram illustrating how two types of exposure control systems for the iris 2 and the AGC circuit 5 are controlled in accordance with illuminance.
In FIG. 2, the horizontal axis indicates the illuminance of the object; and the vertical axis indicates the set value of each of the exposure control systems. Each of the exposure control systems is divided into two control areas, A and B, in accordance with the object illuminance. That is, exposure control is performed by combining two exposure control systems of iris and AGC in accordance with the object illuminance.
In area A, the shutter speed is fixed to 1/60 seconds (1/50 second in PAL), and the gain of the AGC circuit 5 is fixed to 0 dB, exposure control being performed by only the iris 2. In area B, the shutter speed is fixed to 1/60 seconds (1/50 seconds in PAL), and the gain of the AGC circuit 5 is fixed to a full aperture value, exposure control being performed by only the AGC circuit 5. The above-described iris and shutter speed are each controlled, and set at predetermined values, thus realizing the above-described program AE.
In a case in which the shutter speed is changed, for example, from 1/60 seconds to 1/30 seconds, it is possible to practically realize a low-speed shutter of 1/30 seconds by a method in which the image sensor is read at a cycle of two fields to make the accumulation time twice as large. However, even if the shutter speed is changed, since it is fixed in a certain photographic mode, when inadequate-illuminance of an object to be photographed occurs, underexposure may not be compensated even if the iris 2 is fully opened. At that time, since the gain of the AGC circuit 5, necessarily increases, S/N deteriorates, deteriorating the image. Fine exposure control cannot be performed in accordance with slight changes in the photographic environments.